Thermal dye transfer processes have been developed to make prints from a pattern of electronic information signals, e.g. to make print copies of pictures that have been generated electronically by means of a color video camera. To make such prints, the electronic picture can be subjected to color separation with the aid of color filters. The different color separations thus obtained can then be converted into electrical signals which can be processed to form cyan, magenta, and yellow electrical signals. The resulting electrical color signals can then be transmitted to a thermal printer. To make the print, a dye donor element having a repetitive pattern of separate blocks of yellow, magenta, and cyan (and optionally black) dyes is placed in face-to-face contact with a receiving sheet and the resulting sandwich is inserted between a thermal printing head and a platen roller or drum. The thermal printing head is provided with a plurality of juxtaposed heating elements, e.g. such as heat-generating resistors, and can apply heat to selective regions of the back of the dye donor element. For that purpose, the heat-generating elements of the printing head are heated up sequentially in correspondence with the cyan, magenta, and yellow electrical signals, so that dye from the selectively heated regions of the dye donor element is transferred to the receiving sheet and forms a pattern thereon, the shape and density of which are in accordance with the shape and density of the heat applied to the dye donor element.
To achieve the area-wise selective heating of the dye donor element, the thermal printer is designed so that the heating elements thereof can be individually activated by drive signals to effect the thermal transfer of the dye from donor element to receiving sheet.
The image-wise heating of the dye donor element is performed on a line-by-line basis, with the heating elements geometrically juxtaposed alongside one another and with gradual (serial) adjustment of the output intensity of the elements to effect corresponding variation in density of the transferred dye. In contrast, the electrical image data (also referred to as "picture data" or "pixel data") are mostly stored in a parallel organized memory, such as a first-in-first-out (FIFO/fifo) memory or a random access (RAM/ram) memory.
Therefore, a thermal dye transfer printer needs to be able to convert image data signals from a parallel input configuration to a serial output configuration (refereed to for convenience as a P/S conversion). The serial configuration of image data signals resulting from this conversion are used to activate or "drive" the heating elements of the thermal printing head, resulting in recordation of the data on the receiving sheet.
In European patent EP 0 208 919, a P/S conversion is described which uses a comparator device. In this device, the actual image (pixel) data, wherein the binary equivalent of pixel density is represented by bytes consisting of eight bits in parallel, are delivered to one input terminal of a comparator, and the output of a clocked counter which increases incrementally in sequence is fed to a second input terminal of the comparator. The comparator output remains high (expressed symbolically as "logic 1") as long as the magnitude of the counter output at the second terminal is lower than binary equivalent of the pixel density at the first terminal. As soon as the counter output reaches the level equal in binary equivalent terms of the actual pixel density, the comparator output reduces to a low level state (symbolically "logic 0").